Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 122

maxima at the surfactant concentration of 2-5 × cmc because of their solubiliza-
tion to micelles in a water phase (Lee et al. 2004). The modification of soils with
HDTMA Br also enhanced the adsorption of benzene derivatives (Boyd et al.
1988; Gao et al. 2001; Jaynes and Boyd 1991). In the case of positively charged
aluminum oxide, the sorbed SDS and nonionic surfactants have been reported to
enhance the adsorption of many types of organic chemicals (Nayyar et al. 1994;
Park and Jaffé 1993; Valsaraj 1992).
Pesticides
Soil adsorption parameters of some pesticides based on the Freundlich isotherm
together with the effect of adjuvants including surfactants are summarized in
Table 8. The reported effects are highly dependent not only on the chemical class
of adjuvants and their concentration but also those of pesticides. Generally, the
addition of an anionic surfactant reduces the K F value by means of its competitive
adsorption on soil and solubilization to micelles, but a slight increase of adsorp-
tion is sometimes observed (Ye 2003). Either nonionic or cationic surfactant
tends to increase the K F value by the partition of pesticide to the hydrophobic
parts of the soil-sorbed surfactants, especially with a larger extent for the latter
surfactant, but it sometimes decreases at a higher concentration than cmc from
solubilization to micelles in an aqueous phase (Huggenberger et al. 1973;
Iglesias-Jiménez et al. 1996, 1997; Singh et al. 2000, Sánchez et al. 2003b; Sun
and Inskeep 1995). The partition of pesticide to the hydrophobic sites consisting
of the alkyl chains of nonionic surfactants on soil has been confirmed by the posi-
tive correlation of the maximum adsorption amounts of moderately hydrophobic
fungicides with the total hydrophobic carbon number of surfactant (Steurbaut
1994). Locke et al. (2002) reported that the hydrophobic norflurazon (46) is
adsorbed more to soil in the presence of nonionic Triton X-77, but the K d values
of more hydrophilic triazine herbicides are negatively correlated with the carbon
number of the surfactants. The ionizable 2,4-D (26) was adsorbed less to soil
when a surfactant having a lower HLB value was present, showing the impor-
tance of electrostatic interactions (Amonette and O'Conner 1990).
Because the usual pesticide formulation contains an adjuvant consisting of sur-
factant and other components, the aforementioned effects are masked in many
cases. The adsorption profiles in soil, sediment, and clay are almost the same
between an a.i. and its formulation (Beestman and Deming 1976; Beigel and
Barriuso 2000; Ekler 1988; García-Ortega et al. 2006; Singh et al. 2002a), and the
effect of a formulation type is usually insignificant (Chaplain et al. 2001; Cotterill
1988; Mingelgrin and Yaron 1973). However, in the case of urea and carbamate
herbicides, less adsorption to river sediment was reported for the formulations
(Margoum et al. 2006). The effect of surfactant on pesticide adsorption is also
known to be dependent on soil characteristics. The addition of Triton X-77
enhanced the adsorption of primisulfuron (43) to soils with low organic carbon (oc)